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The advent of topological semimetals with peculiar band structure and exotic transport provides an unprecedented material platform that allows exploring novel optoelectronics for circumventing technological bottlenecks. Cd3As2, a three-dimensional Dirac semimetal, represents a hallmark system for studying nontrivial quantum phenomena led by Dirac/Weyl physics. However, controllable growth and device implementation are still in their infancy due to lack of efficient ways to make use of light-induced effects in semimetals. In this study, highly sensitive, low-energy photodetection up to terahertz (THz) band wavelength along with fast response at room temperature has been implemented in an antenna-assisted Cd3As2 planar structure, which is derived from molecular-beam epitaxial growth. It is demonstrated that the THz photodetector based on semimetal Cd3As2 films possesses a responsivity of 0.04 A/W and a NEP value of 430 pW/Hz1/2. Nonequilibrium manipulation of Dirac fermions with thickness-controlled gap phases and an electromagnetic-coupling effect has been well exploited. Our results portray opportunities for developing high-performance, scalable low-energy photodetectors enabled by a Dirac semimetal, which is promising for broadband photoresponses in the highly pursued THz band.

Results of first-principles calculations and experiments focusing on molecular beam epitaxy (MBE) growth of HgCdTe on the alternative substrates of GaAs and Si are described. The As passivation on (2 · 1) reconstructed (211) Si and its effects on the surface polarity of ZnTe or CdTe were clarified by examining the bonding configurations of As. The quality of HgCdTe grown on Si was confirmed to be similar to that grown on GaAs. Typical surface defects in HgCdTe and CdTe were classified. Good results for uniformities of full width at half maximum (FWHM) values of x-ray rocking curves, surface defects, and x values of Hg 1-x Cd x Te were obtained by refining the demanding parameters and possible tradeoffs. The sticking coefficient of As 4 for MBE HgCdTe was determined. The effects of Hg-assisted annealing for As activation were investigated experimentally and theoretically by examining the difference of the formation energy of As Hg and As Te . Results of focal-plane arrays (FPAs) fabricated with HgCdTe grown on Si and on GaAs are discussed.

The influence of different CdTe substrate preheats prior to II-VI molecular beam epitaxial growth on surface stoichiometry and oxygen contamination has been studied using x-ray photoelectron spectroscopy. For 15 min preheats with temperatures ranging from 100 to 450 °C, the cadmium to tellurium ratio and the oxide overlayer thickness of (100) CdTe surfaces was determined. A preheat temperature of 200 °C is found to produce optimum stoichiometry. For lower temperatures the CdTe surface is still tellurium rich, as left after etching with bromine-methanol. For higher temperatures, cadmium evaporates faster than tellurium, leaving again a tellurium-rich surface. The oxygen contamination remains nearly unchanged for temperatures below 250 °C. Oxygen starts to blow off for preheat temperatures above 250 °C, with a steep decrease between 250 and 350 °C. For preheat temperatures higher than 350 °C, the oxygen contamination drops below the detection limit.